If you do have any type of under drive pulleys on there, they need to go. What is the size ratio between your crank and water pump pulleys? You want around 1.2:1, maybe a little more with forced induction.
I had my pulleys made, 7.00’’ crank & 5.875’’ water pump. Had a similar issue to yours few years ago, used those pulleys with a FlowKooler water pump (vane impeller type) big radiator, big block style 160deg thermostat & went back to a mechanical fan. Solved it. You want good airflow and water flow at low engine speeds.